As a child, I once came back from the Bologna Motor show with two cassettes by Mondocorse in hand. What I remember the most from these cassettes filled to the brim with car crashes of all kinds and from all motorsport disciplines, were the ones from drag races.
Parts flying and cars in flames just as the light turned green, smoke and fire and the spectators cheering: I always felt a sort of guilty pleasure in watching those machines blow up like big, expensive firecrackers.
– SBAM! –
Since then I always grew an interest in Top Fuel dragsters and for the surrounding culture. After all, how can you not like the idea of a car with over 12 thousand horsepower capable of covering 305 meters (1000 ft, Top Fuels don’t the quarter mile anymore after Scott Kalitta’s fatal crash in 2008, video HERE) in less than 4 seconds, crossing the checkered flag at over 530km/h?
0 to 530kph in 300 meters.
Top Fuel dragsters accelerate faster than a fighter jet launched by a carrier’s catapult, going 0 to 160 in less than half a second and reaching 450 km/h in the first 200 meters of the race.
To put that into perspective, if you put a Top Fuel dragster against a bullet train and give the train the advantage of a 300km/h rolling start, the dragster starting from still would cross the finish line first.
See ya later!
Get in your car, think of some plausible reason to break quarantine and drive 300 meters. Think about how long you drove, and the time it took you to cover that distance. Now think that a Top Fuel dragster will cover that same distance in 3.6 seconds. That’s less time than it took you to think about it. (at the time of writing, the current world record is 3.659 seconds, clocked by Brittany Force).
A dragster will reach over 460 kph before you have finished reading this sentence
Now, let’s talk engines. They deserve their own paragraph, you’ll see why.
Top Fuel engines, this is madness!
The craziest thing about a Top Fuel’s over 10’000 HP engine is, all that power comes out of an engine developed 50 years ago. This isn’t Formula 1 and there’s no wizardry and electronics at play, just a lot of willpower and pure engineering. If you think that out-of-this-world power figures like that come out of cutting edge tech, you’re wrong: under the hood of these dragsters you’ll find your run of the mill Hemi V8, a model line dating back to the first Elephant 426 engines born in 1964
Well, not exactly your run of the mill Hemi from your uncle’s truck, but you get the point. Starting from a forged aluminum monoblock with cast iron cylinder liners, Top Fuel engines have two valves per cylinder (62mm intake, 49mm exhaust) working in a pushrod layout and a single camshaft placed inside the block, reaching a maximum displacement of 500 cubic inches (that’s over 8 liters, four times the displacement of my puny 4 cylinder!) with a 106mm bore and 114mm stroke.
But how do they manage to squeeze over 10 thousand horsepower out of a road car engine? What’s the trick? The answer is fuel. Because you wouldn’t expect a car only built to reach the highest speed to run on regular pump gas, come on. Instead, to achieve numbers this high, they run on nitromethane.
There’s a bit of chemistry to do to understand the inner workings of these engines, but trust me, it’s worth it.
Okay, so. Nitromethane is the simplest molecule in the family of nitrocompounds: molecules that contain at least one nitro functional group (-NO2). These molecules have a particular characteristic that makes them interesting: the tendency to explode.
You don’t believe me? Here’s an example.
If you don’t live under a rock you must have heard of TNT explosives. Those three letters stand for TriNitroToluene (tri-nitro, starting to see a trend here?), a molecule obtained from toluene (C6H5-CH3), combined with three of the aforementioned nitro functional groups.
Okay, enough chemistry, now we’re back to american V8s and crazy power figures: to make an engine work, you need two things: fuel and air (more precisely, oxygen). To make an engine more powerful, you increase the flow of air to it, either with a bigger intake, forced induction like a turbocharger, etc. You’d do this so that with more oxygen in the engine the fuel burns better, creating a more powerful explosion in the combustion chamber and more force on the piston.
Now, here’s why nitromethane is used. For every kilogram of gasoline burned, you need around 14.7 kilograms of air (21% oxygen), with less it wouldn’t burn at all. Nitromethane instead is partly composed of oxygen, and therefore requires only 1.7kg of air to burn per kilogram of fuel.
This means that with the same amount of air, a nitromethane engine burns eight times the amount of fuel, making loads more power. Even though nitromethane isn’t as energetic as gasoline, the sheer amount of nitromethane you can burn in short time more than makes up for it. Because of all this, the whole point of a dragster engine is to burn lots, lots of fuel.
How much fuel? A dragster’s engine at full power burns around 4 liters of nitromethane per SECOND, so with your foot to the floor the injectors pretty much turn into nitromethane hydrants, just dumping and dumping fuel into the fire.
So, despite nitromethane having less energetic potential per kilogram (11,2 MJ/kg against 44 MJ/kg), the incredibly high amount you can burn in a short time makes it so much more efficient: a regular engine powered by nitromethane makes around 2.5 times the power of that same engine running on standard gasoline, but if we consider that Top Fuel engines aren’t regular engines but engines specifically built to run on nitromethane, the crazy power figures now start to make sense.
Still, even with all the chemistry behind it explained, the power figures still seem absurd and foreign to most of us car enthusiasts. We’re used to seeing one thousand horsepower engines described as the ultimate power goal, it’s hard to imagine ten times that. And it’s also hard to put it on paper: all of these power figures are only estimated, because there is no dyno resistant enough to handle that much power.
But that’s not all. You may have asked yourself “but wait, how does a Top Fuel car cool down an engine with so much power coming out of it? I don’t see huge radiators or cooling systems!”. Well, the answer is, again, fuel. One of the characteristics of nitromethane based fuel (a Top Fuel runs on a mixture of nitromethane and methanol, 90:10) is that it cools the engine on its own.
“What is this, magic?” No, it’s physics. Nitromethane, like every substance (to different degrees), has the characteristic of absorbing heat when changing state between liquid and gaseous (in physics, that’s called Latent Heat or Heat of Transformation). It’s the same reason why sweating makes you feel better when it’s too hot: water, turning into gas, absorbs some heat from your body.
Because of this, the nitromethane-methanol mixture that powers dragsters, which has a way higher latent heat, absorbs high amounts of heat when changing state inside the combustion chamber, cooling off the engine enough to not require intercoolers and everything else an engine needs to cool off (for the happyness of all Subaru open-deck EJ25 owners).
But that’s not all: after a certain temperature, nitromethane can burn and release all its heat (energy!) without needing air, therefore becoming both combustible and oxidizer, and also producing waste idrogen, throwing more burn into the mix.
2 CH3NO2 → 2 CO + 2 H2O + H2 + N2
The mixture of nitromethane and methanol is injected into the engine in liquid state by a group of 42 mechanical injectors: 10 above the supercharger, 16 in the intake manifold, and two more for each cylinder. The amount of fuel injected into the engine is so high that a part of the mixture isn’t burnt quickly enough and ends up in the exhaust, where it gets so hot and unstable that it burns by mere contact with the air outside. This generates an extra force of over 450kg which pushes the dragster both forwards and also downwards, adding “free” downforce to the one generated by the already impressive rear wing, pushing down around 450kg at 500km/h.
Now, you can start to see how crazy these machines are. For every race, a Top Fuel car consumes between 45 and 86 liters of nitromethane mixture. Think about it, 86 liters of fuel just for a burnout and 300 meters. So much for global warming and emission standards, right?
I know what you’re thinking about. You could exploit the power of nitromethane and live something glorious by feeding some to your grandpa’s Fiat Punto, right?
Sadly, it’s not that simple. The engine would crack under all the pressure, both mechanical and thermal. I can assure you however, that in the future songs would be written of your valiant efforts. And it’s not like you can find nitromethane for sale easily on eBay, it’s used in mixtures to fuel RC cars… (please do not try this at home, and if you do don’t hold us responsible for your actions, we’re just explaining things).
Anyways, now that we have uncovered the secret behind the crazy power figures of a Top Fuel dragster, it’s time to understand how those engines are built to run on fuel that once was used on long range ballistic missiles.
To suck in as much air as possible, Top Fuel dragsters run a huge positive displacement Roots type supercharger: at its highest, this supercharger alone absorbs around one thousand horsepower, but can reach pressures around 60 to 65 PSI (up to 4.5 bar, limited by the rules). To give you an idea of how big and powerful this supercharger is, think that it is also strapped onto the engine with two additional Kevlar belts to hold it in place and make sure it doesn’t fly off and land into the grandstands in case of an explosion. That’s not even engineering, that’s madness.
Inside the engine bay, the block, pistons and rods are made out of forged aluminum, while the cross plane crankshaft is made of steel. Even though attempts to use flat plane V8s have been made in the past, the increased vibration meant that the gain in horsepower and lightness was irrelevant as the engine was too unstable to operate. To put into perspective how resistant a Top Fuel crankshaft is, I’m just going to tell you that often, in case of mid-race engine explosions they are known to not break but instead get launched into the air, with the pistons and rods still attached. Just think of how resistant that steel has to be!
Of course, an engine pushing out that much horsepower at that pressure is not going to win any awards in the long term reliability game. Actually, the pressure from one race alone is enough to twist the crankshaft by up to 20 degrees. (even though, they’re designed to still work even with that amount of crankshaft torsion) But this isn’t mom’s daily driver Citroen, and therefore the engine does not need to last: Top Fuel engines are disassembled after every race down to the smallest part, restored and rebuilt for the next race.
An interesting note is that these engines are built to last around 900 revolutions only, 250 of which being in the burnout and 540 in the race. (To achieve these numbers we considered the engine running at the 8400RPM, class limit for the entirety of the race.)
To put down all that power, the engine is linked to the wheels with a single speed transmission with a 3,2:1 ratio, limited by the Top Fuel class rules for safety reasons, and a centrifugal clutch that locks as soon as you push down on the gas, sending the power from the engine to the tires and launching the dragster at an acceleration around 4G of intensity. That’s enough to make your hair fly off if you’re not wearing a helmet!
Only 900 crankshaft revolutions, that’s all it takes. The clutch disks will weld stuck, the crankshaft will twist but not just that: the spark plugs, two per cylinder, will also be completely melted: the current passing through them will be at an intensity of 45 ampere, the same one of an arc welder. After the run, the spark plugs will have reached a temperature of 1400 degrees. You thought that your 1.8 turbo had overheating issues? Cute.
The power that a Top Fuel dragster unleashes when it shoots off the starting line is so high that it shakes the ground up to the second degree of the Richter earthquake scale, and the noise level reaches past 150 decibels (That’s more than a jet engine, or a really close gunshot.)
Now, since I brought Tom Cruise into the question, remember what Harry Hogge told Cole Trickle in one of the greatest racing movies of all time? “Tires, Cole. Tires win races!”. If this sentence is true for NASCAR, it’s even more true for a Top Fuel dragster, since the tires have to take all those 10 thousand horsepower of the roaring V8 and put them to the ground efficiently.
– SWAM SWAM SWAM SWAM –
Since we’ve already tackled every other part, you can already imagine that Top Fuel tires don’t have much in common with the tires on a 2013 Prius. The rear tires of a dragster weigh 21 kilograms and cost 917$ each, and last only 1.5 miles before needing a tire change. Because of the enormous acceleration they’re bolted onto the 16 inch wheel itself, and inflated to between 0.4 and 0.6 bar. The pressure is set that low to make the tire deform once the light turns green and all the power kicks in, creating a larger contact patch.
As the car rolls down the strip, the centrifugal force expands the tire up to a maximum diameter of 38 inches, decreasing friction and lengthening the final drive, increasing top speed.
Even though dragsters seem like dangerous contraptions and would make any elitist or “hipster” car enthusiast smile, there’s a an exceptional level of engineering and culture behind them, a lot more than it may meet the eye at first glance.
Now we just needs to wait until things go back to normal, and then plan a trip to Santa Pod Raceway!